Apparatus for the non-destructive testing of magnetizable objects



g- 1956 F. FOERSTER 2,758,276

APPARATUS FOR THE NON-DESTRUCTIVE TESTING OF MAGNETIZABLE OBJECTS FiledJan. 10, 1952 9-. 2 45 INVENTOR FRITZ FOERSTER 6; g Y F k g W, adww4 WM@741" c ATTORNEYS o 2,758,276 United States Patent Olfice 7,

for fine seams in a specimen, may amount to approximately 1 mm.

2,758,276 The design of such a microprobe calls for the exacttheoretical and experimental clarification of the effect 5 and theoperation of a field strength and field strength gradient meter withhighly permeable cores. The in- Fritz Foerster, Reutlingen, Germany,assignor to Magvestigation of the M harmonic voltage produced by a neticAnalysis Corporation, Long Island City, N. Y., field strength and fieldstrength gradient meter yields a corporation of New Yor the followingrelations:

Field strength meter, no load secondary voltage for Application January10, 1952, Serial No. 265,809 the Mm harmonic 1 Claim. (Cl. 324-34) A mHu =W .w.F.u =.H .h .sin *FI-B The invention relates to f method andapparatus Field gradient meter, no load secondary voltage for for thenon-destructive testing of magnetizable obyects m the M harmonic on onehand for cracks, cavities and similar flaws in the material and on theother hand for hardness, strength (2) um ZWLMF'MAZFILM 005 2 3 1 andcomposition. 0 H

In the non-destructive testing of steel and iron products the magneticparticle method is used extensively. From this it is seen iii? theharmonic is an even Using this method the objects to be tested, aftersuitable haimohis magnetization, are dusted or wetted with ferromagneticExplanation of symbols powder. Proceeding in this manner the powder iscollected at the locations of the flaws by the magnetic stray gig g gfgg ig gii ii i g gf locanons and thereby makes Such F :cross-section ofcore in millimeters The disadvantages of this method are (l) the impos-;igigi gfi ggi gig for Core sibility of distinguishing deep cracks fromshallow sur- Hozfield Strength to bfi measured or average fied facedefects, (2) the limited depth of the flaws detectmen th at the radiantmeter able because a certain minimum gradient of the stray H flfifld menthgdifirence flux is needed to collect the powder, and (3) the slow gspeed of testing. cos

These disadvantages are avoided in using this inven- 4 2H 5 H8 tion byquantitatively measuring directly the flaw stray 2==-' field with helpof energization at suitable frequency and T H1 1- a probe arrangementwhereby the flaw detection, in T i contrast to the usually employedparticle method, is not fl atu ti fi ld Strength dependent upon aminimum concentration of the stray Hkzdistomon of curvature fieldStrength flux. The sensitivity of the quantitative determination 40 ofthe stray flux can be raised by amplification to prac- Referring how toacwmpahying drawing! tically any desired value, and by employing asuitable 1 is a glfllihic fepfeseniaiioh 0f the ideal acoustic devicethe size and shape of the flaw can be Ileiilatioh curve of a p CoilCoreaurally reproduced in pitch and intensity of sound. The 2 is aiepiesshiaiioh of a fixed P y maghsii stray fiux which appears at thelocations of fiaws of held adjacent to a flaw in a iesi p magnetizableobjects preferably after suitable magnetig 3, 6, 7 schematically pi tativ 6 zation is, according to the invention, measured quanti- Positionsused in detecting flawstatively by means of a microprobe whereby the iFig. 8 is a representation of an oscilloscope patter probe produces anelectric voltage which is proportional roll d by a rnicroprobe whichrepetitively describi to the gradient of the stray fiux, and thereby tothe a P h adjacent the P p y Of a st piece at a fixt size of the flaw inthe material. The measuring micro- Cyclic rateprobe consists of 2 coilassemblies connected together one of the cchdiiicns Which ispafiicilialiy imports and containing highly permeable col-e5 ith lmagfor the reliable operation of the meter which indica netic saturationvalues, which are energized by an alfield strength fieid gradient, is pp y bsiwi ternating current, and may be connected to an amplithe fieldstrength, or the field strength gradient and fier, Instruments are wellknown in the prior art which resultant voltage. TO obtain this conditionit is [16-2 use a transformer including primary and secondary windihs'flings wound on a highly permeable core with a saturating alternatingcurrent applied to the primary to devel- 6 (3) 0 even harmonics in thesecondary in proportion to 0 aiiy unidirectional flux. The Beach PatentNo. 2,410,039 5 53;: hfigg i g z gfii i p g g Curve of issued October29, 1946 and the Beach Patent No. 2,47 6,- p y is g y 273 issued July19, 1949, for example, show such instru- 4) t merits. Such instrumentsuse measuring units of rela- H tive-ly large dimensions to measure theearth magnetic 85 where 4 ,1, i the Saturation magnetization held or adisturbance of the earth magnetic field, while this invention uses amicroprobe the core dimensions of From Equation 3 which amount tofractions of a millimeter to a few milli- (5) f 1. f s meters only.Through the use of such a small meas- T M uring unit it is possible tomake a quantitative measure- The apparent permeability fter heavydemagneti:

merit of a stray magnetic flux the effective range of which (short core)is dependent only upon the geometrica dimensions and not the materialitself. For short cores does the magnetic particle test. Usually thedeeper the the permeability may be written: flaw the more gradual thegradient.

L2 Another advantage of the invention is the fact that the microprobecan be effectively adapted to the test 6) specimen for various testpurposes on one hand by suitably l l'. g 3 arranging the two separatetestcoil assemblies with re- 2 4 F gard to each other and on the otherhand by suitably Equation 6 for short core length can be replaced by thedisposing I Probe itself with regard to the test Specimenapproximateequation Pig is a so ernatic picture of a fixed polaritystray 1 L2 10magnetic held 10 at the location of a flaw 11 in the (7) ==F materialbeing tested 12. This field at point A has an where microprobe a strayflux with flow characteristic is indi- =1enuth of core in millimeterscated because the microprobe reacts only upon stray I :conbstant ofinduction H fluxes which change their direction, as at point andCombinin Equations Sand 7 B, with respect to the two pickup or detectingCOllS- i 0 4 W 7 5 4 T Figure 3 shows a gradient microprobe 13 locatedabove 8 the flow 11 with parallel'arranged coil sets or assemblies (8)HI 1 l o 14 an 15 perpendicular to; the surface of the test ob ectW1=number f turns f primary i2. In this combination the tangentialgradient of the l=1engfl1 of il i i normal or perpendicular field 18measured because the '=Strength f primary Current probe only measuresfields or field gradients which be m the longitudinal direction of theprobe in this case'the In addition it IS necessary to compute a certainmaxr- I d includes means connected to the primary winding 14a mansions gength of 6 mcgease 1 or passing an alternating current therethrough and8 require ampere turns Per cm Wm mg eggth a means for responding to aneven harmonic voltage coma funqlon of the core length L can be computefrom ponent developed in the secondary winding 14b. The Equation coilassembly 15 is of identical construction and the even 9 i W T, 7harmonic voltage components developed in the secondary windings of theassemblies 14 and 15 are compared and 1 L2 #0 Equation 9 states thatreducing the core length L for applied to {in mdma.t0r Such anarrange-11pm instance to /2 must increase the current for a fixed numgifigg 325 2 s; gg zz g g $2 2 9; 3 3 gg liiggz ig gzi ggi i i l gcurrents should have peak values at least 1 5 t1mes those heat losses 16times. Equation 9 states that for a given g g fifi i gi gffz; z gg fig gi g core material a loW limit exists for the shortening of the 40 d y fh f f f probe length by the heat loss. Qfii with p i o t e or if J a a ymeans of these computations and considerations it 3: ggf g g g g gg i ig z g i 2:3 is possible to establish the most favorable design and are rW gn rangement of the probe. Particularly it can be stated 7 versedimensions thereof.

that imcroprobigs wlthout excesslle heat. losses can only Fig. 4 shows aprobe arrangement with the transformer be produced with help ofmaterials which have a very Saturation Value Ts By Suitable addition ofnew coils 14 and 15 located above each other parallel to theFerromagnetic alloy components of the core material of Surfiice of thetest piece Such an arrangement Is my low values of saturationmagnetization. The alloy 10 which are caused by Internal Stresses andStructural Down as npermanoy is one Such materials with 10W defects suchas soft spots, stray fields parallel to the suraturation values is itpossible to manufacture micro ace and so.on.(as renresentld at robes thedimensions of which are comparable with one apphcauO-n of microprobemeasure 1e effective extensions of the magnetic stray fields which mentof the Vemcal lent of the veftlca component cur at the surface of flawson ferrous materials. (Of a stray field 10) Is musnated by Fig 5 ere thesing a microprobe of this invention it IS posslble to piobe 13 sdlsposed nominal to the surface of the.test assure quantitativelymagnetic stray fields (gradients) piece 12 with the sets of co1ls 14 and15 arranged coaxially rich for suitably magnetized specimens, occur atsuch above each other T i f arrangement used for ations where theuniformity of the material has been the spewed magnenc pomt method ncase turbed by cracks, Inclusions or cavmes It is 0 cording to theinvention, a suitably formed magnet the terial as to the manner 1n whichthe speclmen 1s ma pole. of which Is Sharp y pomted ls pressed agamstthe ized, as long as a component of the magnetic flux 1s specimen andafter removal thereby forms c1- ight angles to the defect, 1s ofsufiicient strength and men a pom: type magnetic polethe Strendgth 0 2gsists long enough to permit subsequent measurement only dependent uponthe coercive force He to e lg dema netizin effect of the free surface.The stren nspection by the microprobe. For many ob ects such g gth enmeasured by means of e apparatus 18, 16 above the current runs parallelto the flaw, the ree ibe 7 The arrangement of the microprobe 13 above amag- The effective penetration, viz., detection depth, netrc pointinduced in the test specimen 12 therefor I using the microprobe is about10 times greater permits the measurement non-destructively, andindependfor the already mentioned magnetic particle method ent of theshape of the test specimen 12, of an important sting previouslyemployed, because the microprobe material characteristic and in thismanner to evaluate :invention responds to more gradual gradients thanhardness, depth of case and grade. The specific shape of the specimen 12is of no importance in this case because the area extension of the pointpole is insignificant compared with dimensions, curvatures, etc. of thework specimen 12. The point magnetization in addition permits themeasurement of the mentioned properties at different spots of the samespecimen. Inductively hardened parts like car axles, steering wheelshafts and so on thus can be investigated in this manner for soft spots.

In the same way according to the invention it is possible to employ theprobe for the rapid measuring of the residual magnetism remaining in atest piece after magnetization. To do this it is only necessary to movethe magnetized specimen past the probe at a fixed distance. The magneticfield of the test specimen at the location of the probe is determined bythe volume of the test specimen and its coercive force. In view of thefact that the coercive force is proportional to the hardness of manytechnical alloys it is possible to build, with help of the probereferred to in this invention, various apparatuses which permit a fullyautomatic sorting according to hardness of steel parts like drills,rollers, nuts, needles and the like. For this purpose the magnetizedparts to be tested are moved past the probe either by means of a movingbelt or by sliding them down a V-shaped incline and after measurementare automatically graded into hardness groups by means of traps whichare controlled by the measured value. To do this, the probes have to bedisplaced in such combinations that the magnetic field of the earth hasno effect upon the measurements. This can be done by means of twospecific probe orientatrons.

The first possibility is illustrated by Fig. 6. The two sets of coils 14and 15 are disposed above each other parallel to the surface of thespecimen 12 in such connection that a homogeneous earth magnetic fieldis eliminated. On the other hand, the effect of the specimen 12 is muchstronger upon secondary coil 15 than upon secondary coil 14 with theresult that the probe 13 causes the indicator 16 to deflect fornon-uniform hardness or for non-uniform properties of the material.

Fig. 7 shows the second possibility of coil orientation for the residualfield test where the coercive force is a measure of the hardness, and soon, of the test specimen. The two sets of coils 14 and 15 in this caseare located coaxially on both sides of the front end of the testspecimen 12 connected in opposition so that the effect of thehomogeneous earth magnetic field is eliminated. On the other hand, theeffect of the test specimen 12 upon the two secondary coils 14 and 15 isadditive because the field of the test specimen has a reversed directionat the location of each coil. The probe 13 is reacted upon by suchgradient fields but not by the earths magnetic field which has the samedirection and strength at the location of each coil. This is true withrespect to the embodiments illustrated in the other figures as well.

The scanning of rotationally symmetrical work pieces (bars, tubes, andso on) by means of the microprobe of this invention suitably arrangedcan be accomplished in considerably shorter time if the microprobe ismechanically rotated with high speed adjacent to the circumference orperiphery of the test specimen. In testing a hollow workpiece, themicroprobe may be mechanically guided to describe a path adjacent theperiphery of the workpiece, either inside or outside the piece. In thisembodiment the microprobe coils may be the same as those in the otherembodiments, viz., a microprobe as above described is caused to rotatearound inside or outside of the test specimen and therefore the pair ofdetecting or secondary coils periodically pass the flaw in one plane, asthe material moves past the rotating microprobe in another plane, thusscanning all parts of the flaw. Since the primary and secondary coilsrevolve, suitable means must be provided for connecting them to thestationary leads to the oscillator and amplifier in box 18. Such meansare known in the art,

but a. transformer having one rotating winding and one tube screen. Toeffect such correlation it is necessary only to generate a synchronizingsignal at each revolution of the microprobe and to initiate thehorizontal sweeps of the cathode ray beam by these signals. By suitableamplification and shaping of the signals, these signals themselves maycomprise the horizontal sweep voltage. One means for generating suchsynchronizing signals is to rotate with the microprobe a small permanentmagnet past a stationary pickup coil connected through a shaping circuitand amplifier to the horizontal deflection electrodes of the cathode raytube. The stray fields measured by the microprobe rotating around thetest specimen are reproduced as vertical deflection of the electronbeam. The scanning takes place at a rate of 25 to 30 repetitions persecond and produces upon the screen of the cathode ray tube a stationarypattern of all flaws which are encountered on this circumferentialsection of the test specimen. If the test piece moves through themeasuring combination containing the rotating microprobe then there willappear upon the screen the various flaws of the test specimen whilesimultaneously the various flaw-induced voltages may control a relaywhich operates a signal, either audible or visual, if the flaw exceedsan arbitrarily fixed magnitude.

Fig. 8 is the flaw picture on the screen of a cathode ray tube where thecircumference of the test specimen 12 appears as a horizontal line 17.Every point of the circumferential section of the test specimen (a, b,c, d,) is represented by a related point (a, b, c, d) on the horizontalline of the oscilloscope picture and in this way the location of theflaw 11 is fixed. The depth represents the magnitude of the flaw 11 andcorresponds to the amplitude of the indicated deflection 11' while theshape and the extension of the flaw 11 is characterized by the shape ofthe deflection 11 above the horizontal 17.

The particular technical progress claimed in this invention relates tothe fact that a so-called Flaw-Vision- Picture on the screen of thecathode ray tube shows simultaneously (1) the location,

(2) the magnitude (depth), and

(3) the extension viz., shape of the flaw as a quantitative oscilloscopepattern of the circumferential cross-section of the test specimen atthat instant lying in the rotational plane of the microprobe. The pip ofthe oscilloscope pattern may be caused to appear at the center of thesweep when the defect is facing the front of the apparatus, as shown inFig. 8 for example, by angularly adjusting the position of the mentionedstationary pickup coil in which the synchronizing signal is generated.

In the same manner it is possible to test small parts like nuts,rollers, rings and so on, by first magnetizing them, as above described,and then disposing them over a table under which a microprobe isrotating. If one of these parts as disposed on the axis of rotation ofthe microprobe contains a flaw, then the resultant magnetic stray fieldaffects the microprobe and is reproduced on the screen of the cathoderay tube in the manner mentioned above. The measured voltage at the sametime can be employed to automatically sort or eliminate defective partsby means of relay controlled operations.

An important requirement using the probe of this invention for thenon-destructive testing of magnetizable material is the elimination ofthe effect of the earths magnetic field upon the measurements. Toaccomplish this it is necessary that the performance of the two coils ofthe probe should be exactly identical. One requirement for this is thatthe coils should be identical regarding their electrical and'physicalproperties and they are disposed exactly parallel or coaxial to eachother. A microprobe designed along these linesis suited for variousother purposes in addition to the applications alreadymentioned.

Iclaim: In a magnetic field measuring instrument, a core of highpermeability magnetic material having a low saturation flux density anda short length relative to the transverse dimensions thereof with a'maximum dimension of the order of fractions of a millimeter to a fewmillimeters, primary and secondary windings on said core, means forapplying to said primary winding alternating current having a peak valyeat least 1.5 times that required to produce saturation of said core, andmeans coupled to said secondary winding andresponsive to at least oneeven harmonic of the alternating' ffcurrent applied to said primarywinding.

" References Cited in the file of this patent UNITED STATES PATENTSShepardet al. Ian. 1, 1952

